Control system for electric motors



July 21, 1925.

B. W. JONES CONTROL SYSTEM FOR ELECTRIC MOTORS Inventor- Beqjarnin W.Jones, y fi wnhf His Attorney.

' Patented July .21, 1925.

UNITED STATES 1,546,900 PATENT OFFICE.

BENJAMIN W. JONES, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELEC-TRIO COMPANY, A CORPORATION OF NEW YORK.

. CONTROL SYSTEM FOR ELECTRIC MOTORS.

Application filed November 23, 1923. Serial No. 676,660.

To all whom it may concern.

Be it known that I, BENJAMIN W. JoNns,

a citizen of the United States, residing at Schenectady, county ofSchenectady, State a of New York, have invented certain new and usefulImprovements in Control Systems for Electric Motors, 'of which thefollowing is a specification.

My invention relates to improvements in control systems for electricmotors and has for its principal object the provision of means forinsuring the starting of the motor with a predetermined initial torque.My invention is of particular utility when embodied in a control systemfor an adjustable speed motor such as is commonly used in clevator andhoisting service, although my invention also is applicable to thecontrol system of any motor driven apparatus where a smooth and easystarting and control of the apparatus is desirable. In passengerelevator service, a. smooth and easy starting of the elevator car ineither direction of travel is essential to avoid serious discomfort tothe occupants as well as severe strains and shocks to the elevatormechanism. In order that the driving motor may effect such a smooth andeasy starting of the elevator ear, the system of control should limitthe initial starting torque exerted by the motor within a very narrowrange of the torque required merely to balance the carand subsequentlyincrease the to bring the motor up to full running speed. I Furthermore,such a motor control system is readily adapted for dynamically brakingthe elevator car in stopping by providing means for varying theresistance in shunt with the motor armature, which then serves as adynamic braking circuit for the motor, .and applying full field to themotor to increase the dynamic braking efi'ect.

motor torqueand speed to accelerate the elder full field, and finallyweakening the field;

However, in the operation of an elevator provided with such a motorcontrol system it frequently occurs that the. car either overruns orunder-runs the selected floor while stopping under the full fielddynamic braking action of the motor. To bring the car to the selectedfloor the operator may then hastily apply power tothe motor with thefield at full value and the armature shunt resistance at an impropervalue for smooth starting. This results in the motor exerting a startingtorque much greater than desired and the passengers and equipmentconsequentlyexperience asevere jar or jolt.

My invention contemplates eliminating the possibility of produclng sucha jolt or ar by providing a control system in which the motor startingswitch cannot be operated to connect the 'motor to the power sourceuntil after the field is weakened and the armature shunt resistance isset at the pro er value, thus at all times insuring the starting of themotor with weak field and an initial startlng torque of a predeterminedvalue.

For a proper understanding of my invention reference is had to theaccompanying drawing in which the single figure diagrammatically showsthe invention embodied in 'a master controller type of motor controlsystem for the purpose of explaining the principles thereof. The novelfeatures which I believe to be characteristic of my invention arepointed out with particularity in the appendedvclaims.

Referring'to the drawing, the motor 10 having an armature 11 providedwith a suitable armature winding and a shunt field winding 12 maybeconnected by mechanism, not shown, to both drive and dynamically brakean elevator or other apparatus. Motor 10 may be provided with a suitablecommutating field 13 if desired, although such a field winding is notessential in carrying outmy invention. As indicated in the drawing theshunt field winding 12 is connected in series circuit with a fieldregulating resistor 14 and the operating winding of a relay 15 acrossthe current suppl lines 16 and .17. Relay 15 is biased to the c osedposition shown and is desi ned to respond to a field current of afpreeterinined value to open the relay contacts, and the resistor 14 isproportioned to weaken the field current below this predetermined valuewhile it remains in the field circuit.

The resistors 18, '19, and 20 are connected through the contacts ofelectromagnetic switch 21 in a shunt circuit across the motor armature11 and the commutating field winding 13, to supply a variable resistanceload during the dynamic braking operation of the motor and also whenproperly controlled furnish an armature by-pass res1stance of apredetermined value in order to tion by a suitable operating winding aswill more fully appear in connection with the operation of, myinvention. The resistors 19 and 20 are controlled respectively by theelectromagnetic switches 22, and 23,'each of which is biased to theclosed position to establish a shunt circuit around the resistor asshown, and is provided with a suitable operating winding for actuatingthe respective switchcontacts to the open positions. Resistors 24, 25,and 26 comprise the usual motor starting series resistance and arecontrolled respectively by the electromagnetic switches 27 28, and 29,which are biased to the open position, as shown, and are operated to theclosed position to short circuit the resistors by suitable operatingwindings. The several resistors connected in series and shunt with themotor armature are so proportioned that when the armature winding isconnected to the supply lines 16 and 17 with the correspondingelectromagnetic controlling switches in their respective unenergizedpositions shown, the armature current is limited to a predeterminedValue. This value of armature current is such that with the motor fieldcurrent at the weak field value, the initial starting torque exerted bythe motor is just sufficient to move the elevator car slowly andsmoothly from a position of rest.

In order to control the field regulating resistance 14, a relay 30 isprovided with an operating winding 31, one terminal of which isconnected to the resistor 18 and the other terminal is connected to theresistor 26 as.

indicated. Relay 30 is biased to the open positlon, as shown, and theoperating windmg 31 is designed to actuate the relay 30 to close a shuntcircuit around resistance 14 when energized by an appreciable voltagefrom either the current supply lines 16 and 17 during motoring or fromthe motor during dynamic braking operation.

The electromagneticallyactuating reversing switches 32 and 33 arearranged to con nect the motor armature winding 11 to the.

current supply lines 16 and 17 for operation in either direction and areprovided with operating windings 34, and 35 respectively. These switchesare biased to the open position, as shown, and'are equipped with thecustomary mechanical interlock 36 for preventing interference in thecontrol of the motor. It will be observed that one termi nal of each ofthe operating windings 34 and 35 is connected through a commonresistance 37 to the current supply line 17. Theresistor 37 limits thecurrent supplied to either of the operating windings 34 and 35 to avalue which is insufiicient to operate the respective swiches 32 and 33from the open position but which is suflicient to maintain either switchin the closed position. A shunt circuit of negligible resistance aroundthe resistor 37 is arranged to be controlled jointly by the contacts ofrelay 15 and an auxiliary contact mechanism 38 carried byelectromagnetic switch 22 in order to permit operation of the reversingswitches 32 and 33 from the open positions.

For controlling the energizing circuits of the reversing switchoperating windings 34 and 35 as well as the respective operatingwindings of the electromagnetic switches 21, 22, 23, 27, 28, and 29, amanually operated master controller 39 is provided with the cooperatingcontacts and segments 40, 41, 42, 43, 44, 45, 46, 47, and 48. Thesecooperating contacts and segments are arranged so as to engage in apredetermined sequence to energize from the supply lines 16 and 17first, either the operating winding 35 of reversing switch 33 or theoperating winding 34 of reversing switch 32 and then successivelyenergize the operating windings of electromagnetic switches 22, 27 23,28, 21, and 29, when the controller 39 is operated from the off positionin which it is shown through either the control positions 1 to 7 -or 1to 7 respectively.

Operation.

With the parts as thus constructed and arranged and in their respectivepositions shown in the drawing the operation of my improved motorcontrol system is as follows:

Assume that the supply lines 16 and 17 are connected to a suitablesource of current and the motor 10 is at rest. The motor shunt field inenergized through a circuit extending from supply line 16 through thefield winding 12, resistance 14, the operating winding of relay 15 tothe other supply line 17. With the relay 30 in the open position,resistance 14 reduces the current through the shunt field winding 12below the predetermined value previously mentioned and the motor fieldstrength will be at the corresponding weak field value. The relay 15,being operatively; energized only bya field current below thepredetermined value, will remain in the closed position shown. In orderto operate the motor 10 in one direction, assume itto be the updirection of the elevator car for the purpose of clarity, the mastercontroller 39 is. first operated to control position 1. In this positionan energizing circuit for the operating windin 35 of reversing switch 33is completed roin supply line 16 through contact and segment '40,contact and segment 41, operating winding 35, auxiliary contact 38 car.As the motor is thus started, the operating winding 31 of relay 30 isoperatively energized by the voltage of the supply lines 16 and 17 andeffects the operation of the Closure relay contacts to closed position.of relay 30 establishes a shunt circuit about the field re ulatingresistor 14 and the field current is t ereby gradually increased fromthe weak field value to the -full. field value to correspondinglyincrease the motor torque and accelerate the car. It should be notedthat the gradual increase in the motor fieldstrength begins immediatelyafter the motor is started by closure of the reversing switch 33 andcontinues during an appreciable time interval due to the inductiveaction of the shunt field winding. The-increased value of field currentenergizes the operating wind-j ing of relay 15 and causes the circuitcontrolling contacts of the relay to move to open position.

It should be noted that if, atthis point in the acceleration of theelevator car, the controller 39 should be returned to the ofi position,the operating winding 35 of reversing switch 33 is deenergized and'disconnects the motor from the supply lines 16 and 17. The motor 10then operates as a dynamic brake to retard the movement of the elevatorcar. During braking the operating winding 31 of relay 30 is energizedbythe voltage generated by the motor-and maintains the shunt circuitaround the field regulating resistor14 closed to efiect full fielddynamic braking action of the motor.'.

It will be evident that before either of the reversing switches 32 and33 can again connect the motor armature 11 to the supply contacts andagain establish the shunt circuit around the resistor 37. When the motor10 has brought the elevator car almostto a standstill under fullfield-dynamic braking, the voltage generated by the motor then becomesinsufficient to maintain the relay 30 in the closed position. Relay 30there must be.

upon' opens the shunt circuit around the field regulatin resistor 14 andreduces the field current from the full field value to the weak fieldvalue, thus permitting the relay to close according to its bias. Closureof relay 15 again establishes the shunt .circuit around the resistor 37and ermits either of the reversing switch windings 34 and 35 to beoperatively energized when the master controller 39 is operated from theofi position.' 2

However, if after operation of the master controller 39 to the position1 as previously described, the controller 39'is then successively movedthrough theintermediate control positions 2, 3, 4, 5, and 6 to therunning position 7; the electromagnetic switches 22, 27, 23, 28, 21, and29 are successively energized to alternately insert resistance in shuntwith the. motor armature 11 andremove the resistance in series with thearmature, thus gradually accelerating the motor. It will be observedthat upon the energization of the operating winding of electromagneticswitch 21 the shunt armature resistance circut is opened, thus ineffect, inserting infin to resistance in shunt with the motor armature.Although upon the opening of switch 21 the operating winding 31 of relayis no longer energized by the full .voltage ofthe supply lines 16 and17, the voltage drop across the remaining starting resistor fieldcircuit and the motor 10 is accelerated to full running speed under weakfield.

To stop the elevator car by dynamlc;

braking at. any selected floor, the master erating winding of switch 21,this switch closesaccording to its bias and again estab lishes theresistance circuit in shunt with the armature; thus causing the motor tooperate as a dynamic brake. Closure of switch 21 also impresses thevoltage across the motor armature upon operating winding 31 0f .relay.30. This relay'then closes the shhnt circuit around the fieldregulating resistor 14. Since the electromagnetic switch 22 .was-thefirst to be energized, it will be released last to short. circuit theresistor 19 v .as well as close the auxiliary. contact 38.

The motorv 10 then operates as a dynamic brake under full field and witha maximum resistance load to bring the elevator loo car almost to astandstill. When the voltclosing position, the latter opens accordingtoits bias and once more inserts the resistance 14 in the shunt fieldcircuit to reduce the field strength to weak field value. This permitsthe relay 15 to close according to its bias and again cooperate with theauxiliary contact 38 on switch 22 to establish the shunt circuit aroundthe resistor 37.

The starting of the motor to operate the elevator car in the downdirection is accomplished in substantially the same manner as that justdescribed, with the single exception that in control positions 1' to 7the cooperating contact and segments 42 are brought into engagement tocomplete an en ergizing circuit for operating winding 34 of reversingswitch 32 extending from supply line 16 through con-tact and segment 10,contact and segment 42, operating winding 34, contact 38, relay 15 tothe other supplyline 17. Upon response of reversing switch 1 32 themotor armature, with the resistor 18 connected in shunt circuittherewith, is connected in series circuit with the series startingresistors 24, 25, and 26 across the supply lines 16 and 17 for startingwith weak field and the same predetermined initial starting torque inthe opposite direction. The deceleration of the motor under dynamicbraking to stop the elevator car at any selected floor is likewisesimilar to that previously described.

While the above described operation is the usual and normal manner inwhich the motor control system is intended to operate, it should beunderstood that my improved control system insures at all times that themotor is started with weak field and an initial starting torque of apredetermined amount even under unusual and abnormal operatingconditions. For instance, suppose that while the motor is running atfull speed in the down direction the operator should instantly shift themaster controller 39 from the control position 7 to the control position1 to quickly effect the reversal of the motor.' In this event theoperating winding 34. of reversing switch 32 as well as the operatingwinding of the shunt and series armature resistance controlling switcheswould be deenergized and the operating winding 35 of reversing switch 33would be energized. However, it will be seen that the resistance 37prevents sufiicient current from passing through operating'winding 35 toef-' fect the closure of reversing switch 33 at this time. Thus, themotor operates under 'full field as previously described to dynamicallybrake the elevator car until the speedhas been reduced to a value atwhich the voltage generated by the motor is insuflicient to sustainrelay 30 in the closed position. T' hen relay 30 opens as before andinserts the resistance 1 1 in the shunt field circuit to reduce thefield strength to the weak field value. As soon as the field current hasdecreased to the predetermined value at which the relay 15 is no longeroperatively energized, this relay will close according to its bias.After the relay 15 has completed the shunt circuit around resistor 37,winding 35 is energized by the full voltage across the supply lines 16and 17 and operates switch 33 to start the motor with weak field and thepredetermined resistance in shunt to the armature. Thus it will be seenthat a control system embodying my invention always insures that themotor is connected to the source of current with weak field and limitedstarting torque.

As has been previously pointed out, when the master controller 39 isoperated from either direction to the off position, the electromagneticswitch 22 is the last of the armature resistor controlling switches tobe deenergized to permit operation according to its bias. Due to thisfact, I have found in practice that the single auxiliary contact 38 onthe electromagnetic switch 22 is suflicient under all ordinaryconditions to insure that the proper armature shunt and series resistorsare in circuit for starting the motor with a predetermined armaturecurrent. But it will be obvious to those skilled in the art thatadditional auxiliary contacts may be provided for any selected ones ofthe other electromagnetic switches and included in the shunt circuitaround the resistor 37 should it be desired to positively preventoperation of the reversing switches 32 and 33 from the open positionuntil after the particular switch or switches selected have beendeenergized and operated according to their bias. Also it will beunderstood that after the regulating resistor 14 is inserted in theshunt field circuit upon completion of the dynamic braking operation ofthe motor, an appreciable time may be required for the current in theshunt field circuit to decrease to the weak field value due to theinductive action of the shunt field windings. But until the motor fieldcurrent is below the predetermined value, the relay 15 will bemaintained in the open position and prevent completion of the shuntcircuit around resistance 37 and consequently the operative energizationof winding 35 of reversing switch 33 even thou h the contact 38 onswitch 22 is in close position. This feature is of great practicalimportance since it insures that the motor is always started with weakfield.

In accordance with the provision of the patent statutes I have describedthe principles of my invention together with the apparatus which I nowconsider to represent to have it understood that the apparatus shown isonly illustrative and that the in-- vention may be carried out by othermeans.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is,-

1. In a control system, the combination with an electric motor, ofcurrent controllin'g means operable to weaken the motor field and tolimit the motor starting current under weak field to a predeterminedvalue, switch mechanism for connecting the motor to a source of current,-and means for preventing operation of said switch mechanism until afteroperation of said current controlling means to insure starting of themotor with weak field and limited starting current of a predeterminedvalue.

2. In a control system, the combination with an electric motor havingarmature and field windings and regulating resistors in circuittherewith, of switchmechanism for connecting the motor to a source ofcurrent and controlling the said resistors to vary the motor fieldstrength and starting torque,

and means cooperating with the said switch mechanism for insuring thestarting of the motor with minimum field strength and starting torque. v

3.- A control system for an electric motor having armature and fieldwindings and regulating resistors in circuit therewith, comprising astarting switch mechanism for connecting the motor to a source ofcurrent, electromagnetic means for controlling the said starting switchmechanism and the said armature and field resistors, and meanscooperating with the said electromagnetic means for insuringthe startingof the motor 'with weak field and starting torque of a predeterminedvalue. p

4. A control system for an electric motor having armature and fieldwindings and regulating resistors in circuit therewith, c'omprisingswitchingmeans for controlling the saidresistor's to limit the motorstarting torque under weak field to a predetermined value, electricallyactuated switch mechanism for connecting the motor to a source ofourrenty'and means for operatively energizing the said electricallyactuated switch mechanism controlled by the said switching means toinsure the starting of the motor with weak field and starting torque ofa predetermined value.

5. A control system for a reversible. electric motor having armature andshunt field windings and iegulating resistors in circuit therewith,comprising electromagnetically actuated switch mechanism for connectmgthe motorto a source of current, a plurality of mechanically independentswitches for;

controllin the said resistors to vary the motor fie'd strength andstarting torque between minimum and maximum values, and

means dependent upon the operation of said independent switches to theminimum field and starting torque positions for operatively energizingthe said electroma'gnetically actuated switch mechanism.

6. A control system for an electric motor having armature and fieldwindings and resistors incircuit therewith, comprising, anelectromagnetically actuated switch mechanism for connecting the motorto a source of current, a switch for controlling the armature resistorto vary the starting current through the said armature winding, a secondswitch for controlling the field resistor to vary the current throughthe said field winding, a relay connected in circuit with the said fieldwinding and operatively energized responsively to a predeterminedcurrent, and electrical connections for energizing the saidelectromagnetically actuated switch mechanism jointly controlled by thesaid relay and the said armature resistor controlling switch to insurethe starting of the motor with weak field and limited starting torque ofa predetermined value.

7. A control system for an electric motor having armature and'shuntfield windings starting switch mechanism for connecting the motor to asource ofsupply, a resistor in circuit with the operating winding ofsaid mechanism for preventing operation thereof, switching means forcontrolling the said armature and field resistors to weaken the motorfield and limit the motor starting torque under weak field to apredeterminedvalue, and switch mechanism and electrical connectionscontrolled by the said switching means for establishing a shunt circuitaround the said operating winding resistor to permit starting the motorwith weak field and limited starting torque of a predetermined value.

8. A control system for an electric motor having armature andshunt'field windings comprising a resistor in shunt circuit with thesaid armature winding, a reslstor n series circuit with said fieldwinding and a source of current, electromagnetically operated startingswitch mechanism for con necting the motor to the said source ofcurrent, a resistor in series circuit with the operating winding of saidmechanism for preventing operation thereof, switching means for openinga shunt circuit around the said field resistor and closing a shuntcircuit around the said armature resistor to weaken the motor field andlimit the starting torque of themotor under weak field to apredetermined value, and switch mechanism .and electrical connectionscontrolled by said switching means for establishing a shunt circuitaround the said operating winding resistor to permit starting the motorwith weak field and starting torque of a predetermined value.

9. A control system for an electric motor having armature and shuntfield windings, comprising, a resistor in shunt circuit with the saidarmature winding, a resistor in series circuit with said field windingand a source of current, electromagnetically actuated switch mechanismfor connecting the motor armature winding to the said source of current,a switch for closing a shunt circuit around the said armature resistor,a second switch for opening a shunt circuit around the said fieldresistor to reduce the motor field current below a predetermined value,a relay connected in the said field circuit and operatively energizedresponsive ly to current above the said predetermined value, andelectrical connections for energizing the said electromagneticallyactuated switch mechanism jointly controlled by said armature resistorcontrolling switch and said relay for preventing actuation of saidswitch mechanism until after both the closure of said shunt circuitaround the said armature resistor and the reduction of the motor fieldcurrent below the said predetermined value.

10. A control system for an electric motor having armature and shuntfield windings, comprising, a resistor in shunt circuit with the saidarmature windin a resistor in series circuit with said field winding anda source of current, electromagnetically operated switch mechanism forconnecting the motor armature winding to the said source of current, aresistor in circuit with the operating winding of said mechanism forpreventing operation thereof, a switch for opening a shunt circuitaround the said field resistor to reduce the motor field current,a relayconnected in the said field circuit and operatively energizedresponsively to current above a predetermined value, a second switch forclosing a shunt circuit around the said armature resistor to limitthemotor starting torque under reduced field to a predetermined value, andswitch mechanism and electrical connections jointly controlled by saidrelay and said armature resistor controlling switch for establishinga'shunt circuit around the said operating winding resistor after boththe closure of said shunt circuit around the said armature resistor andthe reduction of the motor field current below the said predeterminedvalue to permit starting the motor with predetermined field and startingtorque.

11. A control system for a reversible electric motor having armature andshunt field windings and resistors in circuit therewith, comprisingelectromagnetically actuated reversing switches for connecting the motorto a source of current for operation in either direction, anelectromagnetically actuated switch for closing a shunt circuit aroundthe said armature resistor, a second electromagnetically actuated switchfor opening a shunt circuit around the said field resistors to reducethe motor field current, a master controller for controlling theactuation of the said electromagnetically actuated switches, and means,including a relay responsive to a predetermined current through theshunt field winding and auxiliary switch mechanism operated-by said,armature resistor controlling switch, for preventing actuation of saidreversing switch mechanisms until after the closure of the said shuntcircuit around the said armature resistor and the reduction of the shuntfield current to the said predetermined value.

12. A control system for a reversible electric motor having armature andfield windings adapted for dynamic braking, comprising, a resistor inshunt circuit with the said armature winding, a field regulatingresistor, switching means for controlling the said resistors to stop themotor under full field dynamic braking action and to limit the motorstarting torque under weak field to a predetermined Value, reversingswitches for connecting the motor armature winding to a source ofcurrent for operation in either direction, and means cooperating withthe said switching means and the said reversing switches to insure thestarting of the motor with weak field and a limited torque of apredetermined value.

13. A control system .fora reversible electric motor having armature andfield windings adapted for dynamic braking, comprising a resistor inshunt circuit with the said armature winding, a field regulatingresistor, switching means for controlling the said resistors to stop themotor under full field dynamic braking action and to limit the motorstarting torque under weak field to a predetermined value, electricallyactuated reversing switches for connecting the motor armature winding toa source of current for operation in eitherdirection, and means foroperatively energizing the said electrically actuated reversing switchescontrolled by said switching means to insure the starting of the motor,with weak field and limited torque of a predetermined value.

14:. A control system for a reversible electric motor having armatureand field windings adapted for dynamic braking, comprising, a resistorin shunt circuit with the said armaturewinding, a field regulatingresistor, switching means for controlling the said resistors to stop themotor under full field dynamic braking action and to limitthe motorstarting torque under weak field to a predetermined value,electromagnetically operated reversing switch mechanism for connectingthe motor armature winding to a source ofcurrent for operation in eitherdirection, a resistor in circuit with the operatin winding of saidmechanism for preventing operation thereof, and switch mechanism andelectrical connections controlled by the said switching means forestablishing a shunt circuit around the said operating winding resistorto permit the starting of the motor with weak field and limited torqueofa predetermined value.

15. A control system for a reversible electric motor having armature andshunt field windings adapted for dynamic braking, comprising, a resistorin shunt circuit with the said armature winding, a resistor in seriescircuit with said field winding and a source of current,electromagnetically actuated reversing switch mechanism for connectinthe motor to the-said source of current or operation in eitherdirection, a switch for closing a shunt circuit'around the said armatureresistor to regulate the dynamic braking eflect of said motor andto'limit the motor starting torque, a second,

switch foropening a shunt circuit around the said field resistor toreduce the motor field current below a predetermined value, a .relayconnected in the said field circuit and operatively energizedresponsively to current above the said predetermined value,

and electrical connections for operatively energizing the saidelectromagnetically actuated reversing switch mechanism jointlycontrolled by said first switch and said relay for preventingactuation'of said reversing switch mechanism until after both theclosure. of said shunt circuit around the said armature resistor and thereduction of the motor field current below the said predetermined value.

16. A control system for a reversible electric motor having armature andshunt field windings and adapted for dynamic braking,

comprising, a resistor in shunt circuit with the said armature winding,aresistor in series circuit with said field winding and a source ofcurrent, electromagnetically operated switch mechanism for connectingthe motor armature to the said source of current for .and operativelyenergized responsively to current above a weak field value, a second 7switch; for closing a shunt circuit around the said armature resistor toregulate the dynamic braking effect of said motor and to limit the motorstarting torque under weak field to a predetermined value, and switchmechanism and electrical connections jointly controlled by the saidrelay and the said armature resistor controlling switch for establishinga shunt circuit around the said resistor to permit the starting of themotor with weak field and limited torque of a predetermined value.

17. In a control system, the combination with an electric motor having ashunt field winding and-a resistor in circuit therewith, of switchingmeans for controlling the said resistor to vary the field current ofsaid motor, 'an electrically actuated switch for connecting the motorarmature to a source of supply, a relay for controlling the energizingcircuit of said switch, and electrical connections whereby the saidrelay is operatively energized responsively to the motor field currentfor preventing actuation of the said starting switch.

'18. In a control system, the combination with an electric motor havinga shunt field winding and a field resistor in circuit therewith, ofswitching means for controlling the said resistor to vary the fieldcurrent of said motor, an electromagnetically operated starting switchfor connecting the motor armature to a source of supply, a resistance incircuit with the operating winding of said switch for preventingoperation thereof, and a relay connected in'the said field circuit andresponsive to current below a predetermined value for closing a shuntcircuit around the saidresistance to permit operation of thesaidstarting switch.

19. In a control system, the combination with an electric motor having ashuntfield windin and a dynamic braking circuit, of a switch mechanismfor connecting the motor to a source of supply for operation in eitherdirection and for disconnecting the motor therefrom for dynamic brakingoperation, of a resistor in circuit with the said field winding, anelectromagnetic switch biased to open position and operable to closedposition for controlling the said resistor to vary the field current ofsaid motor and having an operating winding energized responsively to thevoltage of the source during 'moto ring operation and to the voltagegenerated by the motor during dynamic braking 0 ration, and a relayconnected in the said eld circuit and operatively energized responsivelyto field current above a predetermined value for preventing operationIll of the said switch mechanism to connect the motor to the source.

In witness whereof, I have hereunto set my hand this 22nd day ofNovember 1923.

BENJAMIN w. JoN s.

